Optical communications systems are arranged as nodes connected together by optical fiber connections. These connections are formed from multiple spans, with line amplifiers provided between the spans to enable the overall reach of the connection to be increased. In addition to amplification, other measures may be taken at the line amplifier sites, such as dispersion compensation.
The performance of optical transmission systems can be improved by adjusting line parameters at the line amplifier sites, such as signal power to optimise the balance between noise (dominant at low signal power) and non-linear crosstalk and distortion (dominant at high signal powers). In Edge or Metro networks, and in future cost reduced line systems, there may be little directly available information on the provisioned signal power at each amplifier node. There is therefore a need to derive information concerning the characteristics of each span from signals received at the receiving node.
Single channel measurements are desirable for in-service feedback, because in a mesh network, the transmitters and receivers of co-propagating signals may not be co-located, so there may not be access to the data in these channels. Unfortunately, single channel measurements are limited in the extent to which non-linear distortions from different parts of the line can be separately, i.e. spatially, resolved.
Simulations of envelope distortion (assuming that no phase information is available) show that an estimate of average non-linear distortion is possible. However, similar distortions are generated by a small change in the mean provisioned power and by a linear tilt in per-span power along the line. Thus, it is difficult to distinguish between different optical effects using analysis of received signal amplitudes.
More reliable estimates may be possible when phase information is available, for example from a digital coherent receiver, or from a demodulator for differential phase shift keyed (DPSK) signals, but the resolution will be limited by the extent to which chromatic dispersion causes changes in the signal envelope within each span. This is required for identifiably different contributions from different locations along the line.
Various measurements have been proposed for system monitoring, in order to enable system performance to be improved. These include single channel measurements, both at installation and in-service. Multiple channel measurements are also possible at installation and in service, for example cross phase modulation and also four wave mixing measurements.
In-service measurements are desirable because a number of parameters will change over time over the life of the system, and in-service measurement of these will allow performance to be optimised. Examples are provisioned power in each channel, power in each fibre segment, the adding or re-routing of channels, amplifier operating conditions, non-linear distortion (SPM) and non-linear crosstalk (XPM, FWM).
Ideally, in-service estimation should depend only on the performance of the target channel under consideration. Access to the waveforms or bit sequences propagated in neighbouring co-propagating channels may involve additional hardware, software, or management costs, but can enable greater selectivity than purely single channel measurements. In general, it is not desirable to rely on the presence of co-propagating channels, and in a network with optical add-drop capabilities, there is no guarantee that co-propagating channels share the same endpoints. If the variations of fibre dispersion and signal power within the transmission line are known, the magnitude of WDM impairments can be estimated. When information on the waveform and/or data modulation carried by co-propagating waveforms is present, a more detailed and accurate estimation of performance is possible.
There remains a difficulty in obtaining in-service measurements which enable significant improvements in system operation to be obtained.